The difference between the cage lengths is the capacity of the deraileur to manage chain slack. The capacity is expressed in teeth and from shimano's website the medium cage one has capacity of 35T and the long cage one has 41T.

How to calculate the needed capacity - it is the difference between the largest chainring minus the smallest chainring, plus the difference between the largest cog minus the smallest cog. So for 39/26 and 11/36 it is: 39-26 + 36 - 11 = 38T capacity required.

Even if you use shorter cage than required, nothing wrong will happed, as long as you do not shift into the small/small chainring/cog combination. And even if you switch to the small/small combination the pulleys will hit the chain and the chain will be very slack, but nothing should brake.

As far as I know, Shimano makes two XTR Shadow Plus derailleurs -- a long cage (GS) and a "super" long cage (SGS.) I'm pretty sure they recommend the SGS for the 38/26 11-36 "trail" setup. Though my memory may be a little foggy, I did this research last year when making the same purchase and I ended up with the SGS. It works perfectly. Not sure what kind of bike you're putting this on but it's important to consider suspension movement and chain growth. Some dual suspension bikes require pretty long chains so the SGS is even more important. The SGS derailleur is not weird looking in any way (despite the Super! name...) I'm not sure I've ever seen a bike with the shorter SG Shadow Plus -- so that's saying something.... I think the SGS is the "normal" one.

As already mentioned, Shimano doesn't do short, medium and long cages.

I was running the XTR derailleur with the shortest cage on my old bike, with 2 x 9 (38/26 and 11-34). I am quite sure my current bike has the short version too (the latest version with Shadow Plus), and I am running 38/24 with an 11-36 cassette. No problems whatsoever

It'll say (somewhere) on the mech or cage. And FWIW, those capacity specs (35/41T) are very very pessimistic. I've run an old XTR short cage on a triple without any issues, and currently running a medium (GS) on a triple/FS.

It's usually more down to setting it up properly and being *roughly* within the capacity. Worst case (if it's set up properly) is you'll not be able to use a couple of gears, and they are gears that you really wouldn't want or need to run anyway as they are duplicated elsewhere. (small chainring and small sprockets)

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